The above-referenced patent applications disclose aircraft utilizing a free wing configuration. As used in this present specification, a free wing or "freewing" is a wing attached to an aircraft fuselage in a manner such that the wing is freely pivotable about its spanwise axis. This arrangement enables the wing to have an angle of attack which is determined solely by aerodynamic forces acting on the wing. Rotation of the wing, without pilot intervention, induced by changes in the direction of relative wind over the wing surfaces, causes the angle of incidence between the wing and the aircraft fuselage to vary so that the wing presents a substantially constant angle of attack to the relative wind which, in horizontal flight, enables the aircraft to be essentially stall free.
The free wing is free to rotate or pivot about its spanwise axis, preferably located forward of its aerodynamic center. The free wing generally includes left and right wings extending from opposite sides of the fuselage; these wings are coupled together to collectively freely pivot about the spanwise axis. The left and right wings may be adjustable in pitch relative to one another as disclosed in the aforesaid applications, the relevant disclosures of which are incorporated by reference herein. The aircraft may further include rudders and elevators in a tail section or the aft end of the fuselage which may be controlled in a conventional manner for yaw and pitch control, respectively. Further, it will be appreciated that other types of propulsion systems may be utilized, such as counter-rotating propellers, variable pitch propulsion systems, as disclosed in co-pending application serial no. To Be Assigned, entitled "STOL/VTOL Free Wing Aircraft with Variable Pitch Propulsion Means", filed concurrently herewith, and multi-engine arrangements attached to the fuselage.
One of the major advantages of a free wing aircraft is that the aircraft is intrinsically stable; thus, the aircraft is particularly suitable for use as an unmanned aerial vehicle (UAV) where a highly stable platform is necessary and desirable. For example, UAV's are often used by the military as platforms for maintaining sensors trained on a target. Fixed wing UAV aircraft have a high sensitivity to turbulence, particularly at low altitudes; thus a stabilization system is required for onboard sensors to counter turbulence-induced platform motion. The high stability of a free wing aircraft eliminates or minimizes the stabilization problem in a UAV aircraft because the platform itself, i.e., the fuselage, is much more turbulent even in low-altitude, highly turbulent conditions.
The above-referenced U.S. Pat. Nos. 5,340,057 and 5,395,073, incorporated herein in their entirety by reference, disclose different embodiments of a thrust vectoring VTOL/STOL aircraft having a free wing. The first-mentioned patent, '057, discloses a VTOL aircraft generally including a fuselage, a free wing, a propulsion system, and aerodynamic surfaces carried by the fuselage for vectoring the thrust of the propulsion system away from the predetermined direction of flight sufficiently to achieve near vertical flight orientation to establish an angle between the fuselage and the direction of flight. That is, the fuselage is "tilted" relative to the direction of flight. When the fuselage is so tilted, the direction of thrust becomes vectored, that is, the direction is neither vertical nor horizontal, but includes components in both directions. In the embodiment of the '057 patent, the thrust vectoring means is located entirely within the propeller wash. In a second embodiment disclosed in the '073 patent, the thrust vectoring means is not entirely within the propeller wash. Specifically, this embodiment includes a tail section having a horizontal surface which protrudes from the tail section to such an extent that it is not affected by, i.e., is outside of, the propeller wash. Keeping the thrust vectoring means entirely within the propeller wash enables the aircraft to achieve full hover capability.
The operation of both embodiments is similar. Upon take-off, the fuselage of the aircraft is oriented either vertical, in a VTOL aircraft, or near vertical, in a STOL aircraft, so that the thrust vector is entirely or primarily vertical. To transition from vertical to horizontal flight, the pitch of the fuselage is caused to move toward a horizontal orientation. By pitching the fuselage, the thrust vector also inclines from the vertical and thus has a horizontal thrust component. As the fuselage pitches toward the horizontal, the horizontal speed of the aircraft increases, causing the freely rotatable wing to rotate relative to the fuselage in accordance with the relative wind. The effects of the relative wind acting on the freely rotating wings quickly overcome the effects of the airflow over the wings from the propulsion system and, with increasing horizontal speed, the wing develops lift. The aircraft soon transitions into horizontal flight in a free wing flight mode.
To transition from horizontal to vertical flight, the reverse procedure is employed. An "up" elevator command is given to rotate the fuselage toward a vertical orientation with its nose pointed upwardly. Horizontal speed is thus decreased and a vertical thrust vector is introduced. Accordingly, the relative wind changes and the free wing and fuselage ultimately both rotate into a vertical or near vertical orientation.
Unique to the first embodiment is the requirement for launch and landing assists. For instance, the aircraft of the first embodiment must be mounted for launch in a vertical orientation on a launch system. Such a launch system may comprise, for example, suitable guides disposed on the fuselage of the aircraft which engage a launch rail such that the aircraft and rail are directed generally vertically. With the engine started and propeller backwash providing an air flow over the wings, the aircraft lifts off of the launch rail. Catapult assists may be provided. On landing, the aircraft is placed in the vertical orientation and positioned a short distance above a recovery net. When the engine is turned off, the aircraft drops gently into the net.
The second embodiment of the aircraft, the STOL embodiment, permits a very steep descent. In fact, descent angles of 45.degree. or more are not uncommon, particularly for UAV's. While conventional landing gear wheels include some capacity for shock absorption, they cannot damp the landing loads generated at such a steep descent. Hence, when the aircraft descends faster and/or steeper than ideal, the shock loads generated as the aircraft touches the runway cause the aircraft to rebound and bounce off the runway.
Furthermore, one of the most important advantages of a free wing aircraft is the ability to handle turbulence with minimal tossing of the aircraft. Thus, even sensitive equipment can be mounted to the aircraft with relatively simple mounting systems, thereby dispensing with complicated and expensive gimbaling stabilizers used in conventional aircraft. Without an effective shock absorbing and damping systems for landing, however, this advantage is severely compromised.
Thus, it is an object of this invention to provide an improved free wing aircraft which can accommodate extreme shock loads on landing.
Another object of this invention is to provide an improved free wing aircraft having an effective system for absorbing shock loads generated during landing.
Yet another object of this invention is to provide an improved free wing aircraft having an effective system for damping shock loads generated during landing.